Use Z_PARAM_OBJ macros when zval isn't needed

Use Z_PARAM_OBJ macros when zval isn't needed

In some cases, like spl_object_id, the code is simpler but equally efficient
after optimizations.

In other cases, like get_mangled_object_vars(), the compiler can't infer that
the object in the zval won't change.

Closes GH-6567